✨ Add extract file feature

src/interfaces/ISsnFileEntry.ts

@@ -28,6 +28,10 @@ interface ISsnFileEntry {
   comprSize: number;
   /** Decryption keys needed to decrypt the file */
   decryptionKeys: [number, number, number] | undefined;
+  /** Number of the disk where the file is stored (0=.z01, 1=.z02 etc.) */
+  diskNumberStart: number;
+  /** Offset */
+  offset: number;
 }
 
 export default ISsnFileEntry;

src/ssn/decryption/decryptFile.ts

@@ -0,0 +1,17 @@
+import getCrc from './getCrc';
+import int32Mul from './int32Mul';
+
+export default function decryptFile(dv: DataView, length: number, [key0, key1, key2]: [number, number, number]) {
+  for (let j = 0; j < length; j += 1) {
+      let testChar = dv.getUint8(j);
+      const keyPart = (key2 | 2) & 0xFFFF;
+      const decryptedByte = (keyPart * (keyPart ^ 1)) >>> 8;
+      testChar ^= decryptedByte & 0xFF;
+      key0 = getCrc(key0, testChar);
+      key1 = ((int32Mul(((key1 + (key0 & 0xFF)) >>> 0), 134775813) >>> 0) + 1) >>> 0;
+      key2 = getCrc(key2, key1 >>> 24);
+      dv.setUint8(j, testChar);
+  }
+  //if it was decrypted, we skip the first 12 bytes (random encryption header)
+  return new DataView(dv.buffer, 12);
+}

src/ssn/extractFile.ts

@@ -0,0 +1,66 @@
+import * as zlib from 'zlib';
+import ISsnFileEntry from '../interfaces/ISsnFileEntry';
+import decryptFile from './decryption/decryptFile';
+
+class ByteReader {
+  private dvArray: DataView[];
+  private dvIndex = 0;
+  private pos = 0;
+  constructor(dvArray: DataView[], startDvIndex: number, offset: number) {
+    this.dvArray = dvArray;
+    this.dvIndex = startDvIndex;
+    this.pos = offset;
+  }
+  public readByte() {
+    const curByte = this.dvArray[this.dvIndex].getUint8(this.pos);
+    this.pos += 1;
+    if (this.pos >= this.dvArray[this.dvIndex].byteLength) {
+      this.pos = 0;
+      this.dvIndex += 1;
+      if (this.dvIndex >= this.dvArray.length) { throw new Error('Tried to read beyond DataView boundary in extractFile'); }
+    }
+    return curByte;
+  }
+  public seek(num: number) {
+    this.pos += num;
+    if (this.pos >= this.dvArray[this.dvIndex].byteLength) {
+      this.pos -= this.dvArray[this.dvIndex].byteLength;
+      this.dvIndex += 1;
+      if (this.dvIndex >= this.dvArray.length) { throw new Error('Tried to read beyond DataView boundary in extractFile'); }
+    }
+  }
+  public extractDv(length: number) {
+    const dv = new DataView(new ArrayBuffer(length));
+    for (let i = 0; i < length; i += 1) {
+        dv.setUint8(i, this.readByte()); //TODO: refactor this so it is more optimized
+    }
+    return dv;
+  }
+}
+
+/** Extracts the given file from the given DataView array and returns it as an ArrayBuffer.
+ * Will throw an error when end of final DataView is reached.
+ */
+export default function extractFile(file: ISsnFileEntry, dvArray: DataView[]): ArrayBuffer {
+  //use ByteReader for reading a uint8 and seeking forward across DataView boundaries
+  const byteReader = new ByteReader(dvArray, file.diskNumberStart, file.offset);
+
+  if (byteReader.readByte() !== 0x50 || byteReader.readByte() !== 0x4B || byteReader.readByte() !== 0x03 || byteReader.readByte() !== 0x04) {
+        throw new Error('Local file header had wrong magic');
+  }
+  byteReader.seek(22);
+  const localFilenameSize = byteReader.readByte() + (byteReader.readByte() << 8);
+  const localExtraSize = byteReader.readByte() + (byteReader.readByte() << 8);
+  byteReader.seek(localFilenameSize + localExtraSize);
+
+  let dvFinal = byteReader.extractDv(file.comprSize);
+
+  //decrypt file if necessary
+  if (file.decryptionKeys !== undefined) {
+    dvFinal = decryptFile(dvFinal, file.comprSize, file.decryptionKeys);
+  }
+
+  //uncompress file
+  const uncompressedBuffer = zlib.inflateRawSync(dvFinal);
+  return uncompressedBuffer.buffer as ArrayBuffer;
+}

src/ssn/readSsnFile.ts

@@ -60,6 +60,7 @@ export default function readSsnFile(buffer: Buffer): ISsnFileEntry[] {
 
   //Go to start of central dir
   pos -= 20 + centralDirSize;
+  const posCentralDirStart = pos;
 
   for (let i = 0; i < numEntries; i += 1) {
     {
@@ -99,7 +100,9 @@ export default function readSsnFile(buffer: Buffer): ISsnFileEntry[] {
     const extraFieldLength = dv.getUint16(pos, true); pos += 2;
     /** file comment length */
     const fileCommentLength = dv.getUint16(pos, true); pos += 2;
-    pos += 8; //skip disk number start, internal file attributes and external file attributes
+    /** disk number start */
+    const diskNumberStart = dv.getUint16(pos, true); pos += 2; //0=.z01, 1=.z02 etc.
+    pos += 6; //skip internal file attributes and external file attributes
     /** relative offset of local header */
     const relOffset = dv.getUint32(pos, true); pos += 4;
     /** file name (variable size) */
@@ -149,8 +152,12 @@ export default function readSsnFile(buffer: Buffer): ISsnFileEntry[] {
       diffDestLength,
       diffSourceLength,
       diffType,
+      diskNumberStart,
       lastMod: new Date(1980 + (lastModDate >>> 9), (lastModDate & 0x1E0) >>> 5, lastModDate & 0x1F, lastModTime >>> 11, (lastModTime & 0x7E0) >>> 5, (lastModTime & 0x1F) * 2),
       name: fileName,
+      offset: (centralDirOffset > 0) ? //If files are included in this archive, the centralDirOffset will not start from the beginning
+        posCentralDirStart - centralDirOffset + relOffset : //if file is in this archive
+        relOffset, //if we need to look in a disk (e.g. .z01 for this file)
       size: uncomprSize,
     };
 
@@ -167,11 +174,5 @@ export default function readSsnFile(buffer: Buffer): ISsnFileEntry[] {
     fileEntries.push(fileEntry);
   }
 
-  //TODO: read encrypted + compressed file
-  //pos = start of central dir
-  //
-  //fseek(fp, pos - centralDirOffset, SEEK_SET);
-  //...
-
   return fileEntries;
 }